Standard image capture systems will capture images, such as photographic images, that are two-dimensional representations of the three-dimensional world. In such systems, projective geometry best models the process of transforming the three-dimensional real world into the two-dimensional images. In particular, much of the information that is lost in the transformation is in the distance between the camera and image points in the real world. Methods and processes have been proposed to retrieve or record this information. Some methods, such as one based on a scanner from Cyberware, Inc., use a laser to scan across a scene. Variations in the reflected light are used to estimate the range to the object. However, these methods require the subject to be within 2 meters of the camera and are typically slow. Stereo imaging is a common example of another process, which is fast on capture but requires solving the “correspondence problem”, that can be difficult and limit the number of pixels having range data, due to limited feature points.
Another method described in U.S. Pat. No. 4,935,616 uses a scanner-less laser range imaging system to illuminate a scene object with an amplitude modulated laser source. In this patent, the image capture portion of the system includes an image intensifier (in particular, a micro-channel plate) that is connected to modulate the optical signal reflected from the scene object. The distance-related phase shift of the intensity modulation reflected from the scene object can be determined by capturing two images. A first image is captured without modulating the optical signal, and a second image is captured with the received optical signal modulated by the micro-channel plate in phase with the same amplitude modulated frequency as used to modulate the laser source. Both captured images are registered spatially, and the relationship between them is a function of the range to the object in the scene. Once the phase shift has been established, range to the object can be recovered. The second image may be taken by phase shifting either the illumination modulation or the image intensifier modulation. After the images are acquired they are processed on a pixel-by-pixel basis to ascertain the range from the camera to the object for each pixel.
The preferred method of estimating the range in the '616 patent uses a pair of captured images, one image with a destructive interference caused by modulating the image intensifier, and the other image with the image intensifier set at a constant voltage. However, a more stable estimation method uses a series of at least three images, each with modulation applied to the image intensifier as described in commonly assigned U.S. Pat. No. 6,118,946, to Lawrence Allen Ray and Timothy P. Mathers. In that application, the distinguishing feature of each image is that the phase of the image intensifier modulation for each image is unique relative to modulation of the illuminator. If a series of n images are to be collected, then the preferred arrangement is for successive images to have a phase shift of       2    ⁢    π    nradians (where n is the number of images) from the phase of the previous image. However, this specific shift is not required, albeit the phase shifts need to be unique. The resultant set of images is referred to as an image bundle. The range at a pixel location is estimated by selecting the intensity of the pixel at that location in each image of the bundle and performing a best fit of a sine wave of one period through the points. The phase of the resulting best-fitted sine wave is then used to estimate the range to the object based upon the wave-length of the illumination frequency.
A drawback of methods based on the '616 patent is that color information is lost. Unfortunately for color applications, an image intensifier operates by converting photonic energy into a stream of electrons, amplifying the energy of the electrons and then converting the electrons back into photonic energy via a phosphor plate. One consequence of these conversions is that color information is lost. Since color is a useful property of images for many applications, a means of acquiring the color information that is registered along with the range information is extremely desirable.
The system described in the '616 patent may be implemented in relation to a normal camera system; in particular, a standard camera system may be converted into a range capture system by modifying its optical system. The camera may operate as either as a digital camera or a camera utilizing film. In the case of a film based system there are some other requirements that need to be met. These requirements and means for satisfying them are described in the aforementioned copending application Ser. No. 09/342,370, entitled “Method and Apparatus for Scannerless Range Image Capture Using Photographic Film”. As mentioned above, the drawback of such a camera system is its inability to capture a color image. What is needed is a convenient camera system that can capture ranging information without sacrificing the color information that it would otherwise capture.